Inductive position detector

ABSTRACT

Inductive position detector ( 10 ) is described which includes scale ( 20 ) having a longitudinal axis containing a train of magnetic balls ( 22 ), and a transducer ( 30 ) moveable relative to the scale ( 20 ). The transducer ( 30 ) includes means to induce a magnetic field in the scale ( 20 ). Magnetic markers ( 23 ) are provided at axially spaced locations along the scale ( 20 ). As the transducer ( 30 ) is moved along the scale ( 20 ), the pattern of magnetic markers ( 23 ) detected enables the position of the transducer to be determined.

The invention relates to inductive position detectors, particularly ofthe type described in GB1513567 and developments thereof. Such devicesare typically attached to machine tools and can be used to determine theposition of the machine tool head with respect to a workpiece.

Detectors of this type comprise an elongate magnetic element which has aperiodically varying dimension in a direction perpendicular to thelongitudinal axis of the element. The element is typically a train ofsteel balls arranged in a line in point contact. A transducer, whichsurrounds the magnetic element and travels along its length, is used toinduce a magnetic field in the element. The periodic variations in thedimension of the element result in detectable periodic variations in themagnetic field, and as such, provide corresponding periodically varyingsignalling which can be used to determine the relative position of theelement with respect to the transducer. In use, the element and thetransducer are provided on the machine tool to measure, for example, theposition of the tool head with respect to the workpiece.

Currently, actual position is determined with reference to a singledatum, which is external to and separate from the position detector. Thelocation of the datum is also manually programmed into the detector inthe following manner. Prior to programming the datum, the machine tooluser identifies a datum point, typically on the machine tool itself, andmoves the detector to this datum point. At this point, the samesignalling used to determine relative position between the element andthe transducer is analysed and recorded. As the signalling variesperiodically with position, the detector needs to record how manyperiods it has travelled through as well as where it is on a particularperiod.

The datum needs to be reset each time the position detector is switchedoff, for example, at the end of each working day. This can be awkward,lead to errors and be time consuming, particularly if the machine toolis part way through processing a workpiece.

Accordingly, the present invention provides an inductive positiondetector comprising a first member having a longitudinal axis and anelement of magnetic material extending in the direction of thelongitudinal axis and having a periodically varying dimension in adirection perpendicular to the longitudinal axis, and a second membermoveable relatively to the first member along the longitudinal axis andcomprising means to induce a magnetic field in the element, wherein adetectable reference marker is provided on the first member, distinctfrom the element.

As the datum position is fixed within the detector, a user no longerneeds to select a particular datum and go through the manual procedureof programming the datum. Therefore, the possibility of human error inprogramming the datum is reduced. In addition, programming the datum canbe an automated process, not requiring the intervention of the user.Therefore, the task can be completed more quickly.

Prior art devices rely on recording incremental changes in position todetermine actual position with respect to the datum. Thus, if anyreading is missed, this will lead to an error. When the transducer ismoved a significant distance, these errors can add up and becomesignificant.

In particular, when steel balls are used as the magnetic element, anoverall position value is determined by counting the number of ballstraversed from the datum point, referred to herein as pitch count, andalso the position along each ball, referred to herein as bit count. Thepitch count is therefore a relatively low precision measurement and thebit count is a high precision measurement. However, an error in pitchcount may occur if the element is moved too quickly for a change to beregistered by the detector and can lead to significant errors in theactual position measured.

Furthermore, the single reference datum is normally positioned at oneend of the travel of the transducer along the element. Therefore, thetransducer needs to be moved out of position and to the datum position,which can be several metres away, prior to the datum being set and thetransducer being moved back into position. The further the transducer ismoved, the greater the chance of the errors referred to above occurring.

Preferably, the position detector may be provided with plurality ofaxially spaced markers. This allows the position detector to determineits datum without the need to return to a single unique position toreset the datum.

In order to be able to do this, the detector requires some additionalinformation to enable it to determine which marker it is detecting.

One way to do this is to position the markers at different angularpositions around the circumference of the magnetic element, therebygiving different characteristic signals.

In another example, the markers may be positioned with a non-uniformspacing between adjacent markers. With knowledge of the axial positionof each non-uniform marker spacing along the length of the first member,the detector is able to recognise a particular spacing or combination ofspacings of adjacent members as characteristic of a particular positionalong the element. Therefore, position can be determined without theneed to travel a long distance to find the datum.

The non-uniform spacing could be a random or irregular one, providedthat it is known by the detector. However, preferably each intervalbetween adjacent markers is a multiple of a fixed value. Thus, at eachinterval equal to the fixed value, the detector will either detect thepresence of a marker, or will note the absence of a marker. Thisessentially produces a binary code. By suitably arranging the markers, asegment of binary code for each section of the element can be madeunique. The number of digits required to make each binary code segmentunique will depend on the length of the element and the spacing of themarkers. However, the detector will always need to detect onlyrelatively few markers to determine this unique code. The technique alsoproduces signalling which is particularly suitable to digital circuitry.

If the fixed value is equal to the period of the periodically varyingdimension of the element, the signal processing to determine theposition of the detector is made simpler.

The length of each marker is preferably less than the varying elementperiod dimension. Thus, when a marker is detected, this need not giveprecise information about the position of the marker. Instead, it mayprovide a ‘window’ in which the minimum dimension occurs. The preciseposition can then be determined from the signal provided by theinductive element.

When using a plurality of markers, the markers may produce differentcharacteristic signals from one another. Rather than being limited tohaving to detect the spacings between markers, the ability todistinguish between markers makes it is possible for a single marker toprovide more positional information and thus increases the flexibilityof the detector. More positional information can therefore be providedin a given length along the element.

A particularly convenient way of providing different characteristicsignals is for the markers to be magnetic and to be positioned such thatfor some markers the north pole is detectable, while for others thesouth pole is detectable. When the markers are spaced at multiples of afixed value, such magnetic markers can be conveniently used to provide abase three signal, hence reducing the number of markers required toprovide a unique signal.

One way in which the markers can be detected is to pass them under asingle detector. Provided that the rate of relative movement or distancetravelled between the first and second members is known, the position ofeach marker can then be determined. However, preferably, a number ofaxially spaced marker detectors are provided. This allows the presenceof a number of markers to be detected simultaneously with only a shortrelative movement between the first and second members.

In the situation where the markers are spaced at a multiple of a fixedvalue equal to the period of the periodically varied dimension of theelement, it is advantageous to space the detectors evenly at an intervalequal to the period of the periodically varying dimension. With such anarrangement, a maximum relative movement of one period will ensure thateach marker is detected by a marker detector.

In a development of such an arrangement, it would be preferable for eachmarker to produce a signal which is detectable by a marker detector whenthe marker is axially offset from the marker detector. Thus, it would bepossible to determine whether a marker is between two detectors withoutany relative movement between the markers and detectors. This provides atrue absolute position detection in as much as, when the detector isswitched on, it can immediately determine its absolute position withoutrelative movement between the members. The detection of the markers bythe marker detectors enables the detector to set its absolute pitchcount, while the inductive signal produced by the element allows it todetermine the bit count within the pitch. Thus the detector can set aprecision absolute datum.

In a preferred embodiment, the markers are positioned in a sequencecomprising a series of segments of predetermined length, in each ofwhich segments the arrangement of markers is unique. In this way, assoon as the arrangement of markers in one segment has been detected, aunique signal indicating the position is available.

It is preferable if the number of detectors provided is sufficient todetect the arrangement of markers in any one segment without relativemovement between the detectors and the markers. This ensures absoluteposition detection in that the detectors can detect a unique positionsignal even when stationary. In order to achieve this, the number ofdetectors provided is conveniently greater than the maximum number ofmarkers in a segment. In this way, there is always a detectorsufficiently close to each marker in the segment in order to ensure thata strong signal can be detected.

Preferably, the position detector also includes means to determine whichof the plurality of detectors is positioned closest to the markers in asegment so that only those best placed to read the strongest signals areemployed to generate a position. One such embodiment uses aclassification of the bit count, derived from the inductive element, toselect an optimum subset of detectors from the plurality of detectors.

The element of magnetic material may comprise a train of substantiallyidentical spherical balls disposed side by side in point contact andconstrained against relative movement to one another. In such anarrangement, each marker may be conveniently located in a ring nestedbetween adjacent balls.

Preferred embodiments of the invention will now be described withreference to the accompanying drawings in which:

FIG. 1 illustrates a position detector according to one embodiment thepresent invention having a number of markers and a single markerdetector;

FIG. 2 illustrates a position detector according to another embodimentthe present invention having a number of markers and a number of markerdetectors;

FIG. 3 illustrates a position detector according to a third embodiment,which is similar to FIG. 2 but with an alternative configuration ofmarker detectors; and

FIG. 4 illustrates a position detector according to a fourth embodiment,with an array of detectors, and markers capable of giving differentsignals from one another; and

FIG. 5 is a cross sectional view of the scale for a position detectoraccording to a fifth embodiment of the invention.

The position detector 10 as shown in FIG. 1 comprises a scale 20 whichextends longitudinally, and a transducer 30. The transducer 30 encirclesthe scale 20 and is moveable along the length of the scale 20.

The scale 20 comprises a tube 21 of non-magnetic material which houses atrain of magnetic balls 22 in point contact and constrained to preventrelative ball movement. The transducer 30 comprises transmission coils(not shown) and pick-up coils (not shown). The transmission coils areused to induce a magnetic field along the line of point contact of theballs 22 and the pick-up coils are arranged to detect variations in themagnetic field as the balls 22 move relative to the pick-up coils.Detector circuitry (not shown) is used to analyse the signalling to givepositional information. Such devices are well known in the art and onesuch device has been described in GB1513567. In particular, an overallposition value is determined by counting the number of balls 22traversed from a datum point, known as pitch count, combined with theposition along each ball 22, known as bit count. The pitch count is arelatively low precision measurement and the bit count is a highprecision measurement which is absolute within the pitch.

As shown in FIG. 1, a single marker detector 31, such as a Hall effectsensor, is provided in the transducer 30. A plurality of magneticmarkers 23 are provided at axially spaced locations along the scale 20.Each marker 23 is positioned at an axial location level with a point ofcontact between two adjacent balls 22. It will be appreciated from FIG.1 that there is sufficient space for the markers 23 to be positionedhere, typically held in a plastic ring (not shown) which nests betweenbut does not interfere with the magnetic balls 22.

The markers 23 are provided between some of the balls 22 but are notprovided between every ball 22. Therefore, the markers 23 are positionedat intervals equal to multiples of the pitch of the scale 20, the pitchbeing the diameter of the balls 22.

The absence of a marker 23 provides no signal (0) to the marker detector31 whereas the presence of a marker 23 provides a signal (1). Thus, asthe transducer 30 is scanned past the scale 20, at each interval equalto one ball diameter (one pitch) the marker detector 31 detects eitherthe presence or absence of a marker 23 to build up a unique binarysignal which corresponds to a characteristic arrangement of markers 23.

The markers are arranged such that a binary sequence is provided whichcan be viewed as being made up of a series of overlapping segments, eachhaving a given number of digits. A binary sequence can be generated inwhich no segment is repeated within a given possible maximum length. Ifa segment consists of 8 digits, a binary sequence of 2⁸=256 segments canbe generated before repetition of a segment occurs.

By way of example, if the sequence starts with, say, 101100111010 . . ., the first 8 digit segment is 10110011. Moving along one pitch to thestart the next segment, this second segment is 01100111. Moving alonganother pitch, the third segment is 11001110, and so on. Each of thesesegments is unique within the length of the scale 20.

Thus, with knowledge of what the binary sequence is along the scale 20,once the detector 31 has detected any 8 digit segment, it is able toidentify its position relative to the scale 20 by identifying where inthe known sequence that characteristic segment occurs.

A development of the invention is shown in FIG. 2. In this case, anumber of marker detectors 31 are provided. Conveniently, the number ofmarker detectors 31 is equal to the number of digits in the binary codesegment which is required to identify a particular characteristicarrangement. In this case, the binary code segment is 8 digits long andthus there are 8 marker detectors 31. Thus, it is only necessary to movethe transducer 30 for a maximum distance of one pitch length (i.e. oneball diameter) in order to be able to read a whole segment and determinefrom that a position.

In the embodiment shown in FIG. 3, a plurality of analogue detectors 31are provided. These are located at a spacing less than the pitch of themagnetic balls 22. The use of analogue detectors allows the presence ofa marker 23 to be detected even when it is not directly aligned with adetector 31. Also, the strength of the signal detected by the detector31 will vary in inverse proportion to its axial distance from the marker23. Therefore, the detectors 31 are always able to determine thearrangement of markers 23 in the section of the scale 20 to which theyare adjacent without requiring any relative movement between thetransducer 30 and the scale 20. Thus, this arrangement offers trueabsolute position detection.

In a further embodiment illustrated in FIG. 4, markers 23 are used whichcan provide a characteristic signal differing from one another. In thisexample, magnetic markers 23 are used and are positioned as such thateither the North pole or the South pole is detectable. Thus, thedetectors 32 will detect either the absence of a marker (0), a Northpole (1) or a South pole (2) even when the detectors are not directlyaligned with the markers. This provides a base 3 code, rather than abinary code.

As mentioned above, a segment of code with a given number of digits mustbe chosen to represent a particular position. To provide absoluteposition detection the code segments along the whole length of the scale20 must not repeat. This condition can be satisfied if a suitablemathematical algorithm is used to produce a pseudorandom sequence whichin turn defines the marker positions. The advantage of a base 3 coderather than a binary code is that for a code segment of a given numberof digits, a longer pseudorandom sequence can be generated beforerepetition of a particular segment occurs. Therefore, a longer scale 20can be provided.

However, it is undesirable for the code segment to have too great anumber of digits because as the number of digits increases a greaternumber of detectors 31 is required to read a segment without relativemovement of the detectors 31 along the scale 20.

In a preferred embodiment, the chosen code segment is 6 digits inlength. A base 3 sequence of 3⁶=729 unique code segments can thereforebe generated.

In order to be able to detect a 6 digit code segment at any point, aplurality of detectors 31 closely packed in an array 32 is used which issufficient to span more than six of the magnetic balls 22 in the scale20. Furthermore, in order to most clearly detect a marker 23 (or theabsence of a marker) even if a detector 31 is not located exactlyaligned with a marker 23, it is preferable if the array 32 contains anumber of detectors which is much greater than the number of digits inthe code segment to be detected. Using the high precision bit countmeasurement which indicates the exact position over a ball 22, thosedetectors 31 in the array 32 which are best positioned to detect thestrongest signals from the markers 23 in a particular segment can bechosen to provide the code segment, while the signals from the remainingdetectors are filtered out. Typically, the bit count (i.e. theindication of position over a ball) is classified into a series ofgroups, and a particular subset of detectors 31 is associated with eachgroup. Thus, if the bit count falls within a given group, theappropriate subset of detectors 31 is chosen to detect the signals fromthe markers 23.

In a preferred embodiment, the array 32 comprises 16 detectors 31 and islong enough to pick up the signals across 7 pitch-lengths. Therefore,two 6 digit code segments are detectable. Again, using the highprecision bit count measurement to indicate the exact position over aball 22 it can be determined which of the two consecutive 6 digit codesegments the array 32 is closest to and so best placed to detect. Thetransition from one 6 digit code segment to the next is therefore asprecise as the bit count measurement.

By ensuring that an entire code segment can be read by the array 32 atone time without the need to move the transducer 30, this embodimentprovides absolute position detection, i.e. the device is capable ofdetermining its exact position at all times, even if power is removedand restored with movement occurring during the absence of power.

FIG. 5 illustrates another embodiment of the invention in which, at anumber of locations along the scale 20, one or more markers 23 areprovided at differing angular positions around the circumference. Inthis example 3 markers 23 are shown at angles of 0°, 45° and 90°.However, fewer or more markers could be provided at other angularpositions. According to the arrangement of markers 23 a differentcharacteristic signal is provided. Thus, each characteristic signalprovides a datum point on the scale 20.

Markers 23 may be arranged at relatively few axial locations along thescale 20 to provide a series of datums, so that a detector 31 need onlybe moved a relatively short distance to identify a datum. This does notprovide absolute position detection but is a comparatively simplearrangement.

Alternatively, a greater number of markers 23 could be used at each of agreater number of locations so that characteristic signals sufficient toindicate position could be detected at any point. Such an arrangementwould be more complex but could provide absolute position detection.

It should be noted that, in all of the above mentioned cases, theinformation provided by detecting the position of the detectors 31provides a low precision detection but gives information about a uniqueposition. Thus, by resolving the induced signal from the magnetic balls22 and the unique signal produced by the markers 23, the positiondetector is able to determine its unique location with high precision.

1. An inductive position detector apparatus comprising: a first memberhaving a longitudinal axis and having an element of magnetic materialextending in the direction of the longitudinal axis and having aperiodically varying dimension in a direction perpendicular to thelongitudinal axis; a second member movable relatively to the firstmember along the longitudinal axis and having means to induce and detecta magnetic field in the element, the magnetic field having variationscorresponding to the periodic variations in the dimensions of theelement; and a detectable reference marker provided on the first member,distinct from the element.
 2. The position detector as claimed in claim1, comprising a plurality of markers.
 3. The position detector asclaimed in claim 2, wherein the markers are positioned with anon-uniform spacing between adjacent markers.
 4. The position detectoras claimed in claim 3, wherein each interval between adjacent markers isa multiple of a fixed value.
 5. The position detector of claim 4,wherein the fixed value is equal to the period of the periodicallyvarying dimension of the element.
 6. The position detector as claimed inclaim 2, wherein the markers produce different characteristic signalsfrom one another.
 7. The position detector as claimed in claim 6,wherein the markers are magnetic and are positioned such that, for somemarkers the north pole is detectable, while for others the south pole isdetectable.
 8. The position detector as claimed in claim 1, wherein theposition of the marker coincides with a minimum dimension of the varyingelement dimension.
 9. The position detector as claimed in claim 2,wherein the position of each marker coincides with a minimum dimensionof the varying element dimension.
 10. The position detector as claimedin claim 1 comprising a number of axially spaced marker detectors. 11.The position detector as claimed in claim 10, wherein the markerdetectors are evenly spaced at an interval equal to the period of theperiodically varying dimension of the element.
 12. The position detectoras claimed in claim 10, wherein each marker produces a signal which isdetectable by a marker detector when the marker is axially offset fromthe marker detector.
 13. The position detector as claimed in claim 2,wherein the markers are positioned in a sequence comprising a series ofsegments of predetermined length, wherein for each segment thearrangement of markers is unique.
 14. The position detector as claimedin claim 13, wherein the number of detectors provided is sufficient todetect the arrangement of markers in any one segment without relativemovement between the detectors and the markers.
 15. The positiondetector as claimed in claim 14, wherein the number of detectors isgreater than the maximum number of markers in a segment.
 16. Theposition detector as claimed in claim 15, further comprising means todetermine which of the plurality of detectors is positioned closest tothe markers in a segment.
 17. The position detector as claimed in claim1, wherein the element of magnetic material comprises a train ofsubstantially identical spherical balls disposed side by side in a linein point contact and constrained against relative movement to oneanother.
 18. The position detector as claimed in claim 17, wherein themarker is located in a ring nested between adjacent balls.